Many ways. If the tube is blocked or partially blocked, an egg can 'snag.' if the tube is open but doesn't move properly the egg/embryo can GT stuck. Some believe, and it's likely true, that an abnormal embryo may sometimes stick in a tube for reasons unrelated to the tube.

Happens all the time. Eggs are picked up by the fallopian tube and if not fertilized melt away. A fertilized egg, or embryo, can get caught in a blind cul de sac in the fallopian tube, possibly resulting in an ectopic, or tubal pregnancy.